Diet and biting behavior Adult tabanids feed on nectar and plant
exudates, and some are important
pollinators of certain specialised flowers; and certain
Pelargonium. Both males and females engage in nectar-feeding, but females of most species are
anautogenous, meaning they require a blood meal before they are able to reproduce effectively. To obtain the blood, the females, but not the males, bite animals, including humans. The female needs about six days to fully digest her blood meal and after that, she needs to find another host. Both male and female flies are strongly attracted to polarized light reflecting off of dark objects, and in females it appears to be the primary determining factor in guiding them to a host for feeding, as males and females can readily be attracted by black inanimate objects with a smooth and reflective texture. In observations of wild tabanids, the tilt of a surface relative to the direction of gravity also significantly affects their behavior. The most attractive surfaces were ones parallel to the ground. Males and females differed in how they reacted to ever-increasing tilt. Males became less likely to land the higher the surface was tilted. Females initially also became less likely to land the higher the tilt became, but the trend reversed at a tilt of 75° degrees, hit a secondary (albeit much smaller) peak when the surface was perpendicular to the ground, and then rapidly declined again. Neither males nor females were attracted to "overhanging" surfaces a step above 90° to any meaningful degree. '' mouthparts: The sharp cutting stylets are on the right, the spongelike lapping part in the centre. The mouthparts of females are of the usual
dipteran form and consist of a bundle of six
chitinous stylets that, together with a fold of the fleshy
labium, form the
proboscis. On either side of these are two maxillary palps. When the insect lands on an animal, it grips the surface with its clawed feet, the labium is retracted, the head is thrust downwards and the stylets slice into the flesh. Some of these have sawing edges and muscles can move them from side-to-side to enlarge the wound.
Saliva containing
anticoagulant is injected into the wound to prevent clotting. The blood that flows from the wound is lapped up by another mouthpart which functions as a sponge. Bites can be painful for a day or more; fly saliva may provoke allergic reactions such as hives and difficulty with breathing. making them a particular nuisance near swimming pools. Since tabanids prefer to be in sunshine, they normally avoid shaded places such as barns, and are inactive at night. The striped hides of
zebras may have evolved to reduce their attractiveness to horse-flies and
tsetse flies. The closer together the stripes, the fewer flies are visually attracted; the zebra's legs have particularly fine striping, and this is the shaded part of the body that is most likely to be bitten in other, unstriped equids. More recent research by the same lead author shows that the stripes were no less attractive to tabanids, but they merely touched—and could not make a controlled landing to bite. This suggests that a function of the stripes was interfering with
optic flow. This does not preclude the possible use of stripes for other purposes such as
signaling or
camouflage. Another disruptive mechanism may also be in play, however: a study comparing horse-fly behaviour when approaching horses wearing either striped or check-patterned rugs, when compared with plain rugs, found that both patterns were equally effective in deterring the insects.
Reproduction Mating often occurs in swarms, generally at landmarks such as
hilltops. The season, time of day, and type of landmark used for mating swarms are specific to particular species.
Eggs are laid on stones or vegetation near water, in clusters of up to 1000, especially on emergent water plants. The eggs are white at first but darken with age. They hatch after about six days, with the emerging larvae using a special hatching spike to open the egg case. The
larvae fall into the water or onto the moist ground below.
Chrysops species develop in particularly wet locations, while
Tabanus species prefer drier places. The larvae are legless grubs, tapering at both ends. They have small heads and 11 or 13 segments and moult six to 13 times over the course of a year or more. In temperate species, the larvae have a quiescent period during winter (diapause), while tropical species breed several times a year. In the majority of species, they are white, but in some, they are greenish or brownish, and they often have dark bands on each segment. A respiratory siphon at the hind end allows the larvae to obtain air when submerged in water. Larvae of nearly all species are
carnivorous, often cannibalistic in captivity, and consume
worms, insect larvae, and
arthropods. The larvae may be parasitized by nematodes, flies of the families Bombyliidae and Tachinidae, and Hymenoptera in the family
Pteromalidae. The pupae are brown and glossy, rounded at the head end, and tapering at the other end. Wing and limb buds can be seen and each abdominal segment is fringed with short spines. After about two weeks,
metamorphosis is complete, the pupal case splits along the thorax, and the adult fly emerges. Males usually appear first, but when both sexes have emerged, mating takes place, courtship starting in the air and finishing on the ground. The female needs to feed on blood before depositing her egg mass. Horseflies can lay claim to being the fastest flying insects; the male
Hybomitra hinei wrighti has been recorded reaching speeds of up to when pursuing a female.
Predators and parasites ,
Stictia carolina, catches horse-flies to provision its brood in a nest. Eggs are often attacked by tiny
parasitic wasps, and the larvae are consumed by birds, as well as being paratised by
tachinid flies, fungi, and
nematodes. Adults are eaten by generalized predators such as birds, and some specialist predators, such as the
horse guard wasp (a
bembicinid wasp), also preferentially attack horse-flies, catching them to
provision their nests.
As disease vectors Tabanids are known
vectors for some
blood-borne bacterial, viral,
protozoan, and
worm diseases of mammals, such as the
equine infectious anaemia virus and various species of
Trypanosoma which cause diseases in animals and humans. and tabanids are known to transmit
anthrax among cattle and sheep, and
tularemia between
rabbits and humans. Blood loss is a common problem in some animals when large flies are abundant. Some animals have been known to lose up to of blood in a single day to tabanid flies, a loss which can weaken or even kill them. Anecdotal reports of bites leading to fatal
anaphylaxis in humans have been made, an extremely rare occurrence.
Management Control of tabanid flies is difficult.
Malaise traps are most often used to capture them, and these can be modified with the use of baits and attractants that include carbon dioxide or
octenol. A dark shiny ball suspended below them that moves in the breeze can also attract them and forms a key part of a modified "Manitoba trap" that is used most often for trapping and sampling the Tabanidae. Cattle can be treated with pour-on
pyrethroids which may repel the flies, and fitting them with insecticide-impregnated eartags or collars has had some success in killing the insects. ==Bites==